1. Field of the Invention
The present invention relates to semiconductor devices and, more particularly, to a low temperature method of filling contact holes or vias with a low melting point aluminum material and subsequently depositing a second layer dopant for diffusion into the aluminum-filled contact hole or via to form an alloy therein.
2. State of the Art
As semiconductor device dimensions shrink, both gap-fill and planarity of the dielectric films become increasingly important. These challenging gap-fill requirements have initiated and stimulated a search for new processes and materials. Many of these devices, such as advanced ultra-large scale integrated (ULSI) devices, utilize elaborate, multi-level metallization schemes to enhance performance and achieve functional integration. As these device dimensions shrink, intra-lead capacitance becomes a major limiting factor in determining the total interconnect capacitance. Use of multi-level metal structures incorporating low dielectric constant materials is therefore necessary to limit the impact of capacitance on power, cross-talk, and RC delay of dense, deep sub-half micron interconnects.
Due to the ease of its integration therein, aluminum materials are a preferred material for contact/via resistances, having fewer overall process steps and improved electromigration performance. While aluminum reflow has been used for filling contacts and vias having widths equal to or smaller than 0.5 xcexcm, aluminum reflow processes have not been widely accepted due to the higher deposition temperatures required in comparison to filling processes employing metals or alloys having lower melting-point temperatures than aluminum materials. Additionally, aluminum reflow processes are usually ineffective in completely filling contacts and vias having high aspect ratios, that is, contacts and vias having a high ratio of length or depth of a hole or via in relation to the preplated diameter of the contact or via.
Various methods of spreading aluminum or other conductive film on the principal surface to fill the contact holes are already in practical use. These methods include a high temperature sputter method, a bias sputter method, and a reflow after sputter method. A major disadvantage of these conventional aluminum reflow processes is the sensitivity of reflow to surface conditions, hole profile and the type of substrate material. For example, conventional hot sputter deposition and/or reflow processes rely on the diffusive mobility of the atoms. Reflow characteristics are adversely affected by higher contact/via aspect ratios and the typical protrusion of sputtered barrier layers at the hole entrance, making consistent global filling difficult to achieve. Other detriments to complete filling include the presence of spin-on dielectrics and the associated out-gassing from the vias during the reflow process. Global filling is of particular concern for sub-half micron applications since a feasible aluminum reflow technology must be capable of achieving at least an equivalent yield and reliability as compared to conventional technologies, such as a tungsten plug process.
To alleviate some of these problems, a high pressure ( greater than 700 atm) forced fill Al-plug process has been used for sub-half micron contact and via hole filling. This process typically consists of a bake, soft sputter etch, barrier deposition and aluminum plug formation. The aluminum hole filling is achieved via a two step process. As shown in FIGS. 1 and 2 (representing a section or segment of a semiconductor wafer 30), aluminum is applied to insulating layer 24 (typically comprising a dielectric such as SiO, boron nitride, and silicon nitride deposited over a substrate 20) through a conventional sputter deposition technique at about 400xc2x0 C. Prior to the deposition of aluminum, holes or vias 25 are created (e.g. by etching) in insulating layer 24. The deposited aluminum fills or bridges the mouth of each hole 25 with metal alloy layer 22. However, due to the high aspect ratio of the formed hole and the inherent surface tension of aluminum alloy layer 22, void 26 usually forms inside each hole below the filled or bridged mouth. The wafer is then transferred under vacuum to a so-called FORCE FILL(trademark) Module, shown schematically in FIG. 7, consisting of a high pressure chamber 80 with two radiant heaters 82 for controlling the temperature of wafer 84. Outlet port 88 is connected to a vacuum and controls pressurization and removal of gases from chamber 80. Inlet port 86 is connected to a pressurized source of gas, such as argon, for pressure regulation within chamber 80 and introduction of a precursor for plasma formation. The deposited aluminum is then forced into the holes by pressurizing the chamber, usually to about 760 atm, with argon while maintaining the temperature at about 400xc2x0 C. As a result of the forced external pressure (represented by arrows 27 in FIG. 2), the aluminum bridge over hole or via 25 is deformed or extruded inwardly to accomplish complete hole filling, as shown in FIG. 2.
For purposes of the forced fill process, use of a low melting-point aluminum alloy (e.g. alloys of aluminum containing between about 10% and about 60% copper), which flows at reduced temperatures, is preferred over pure aluminum or high melting-point aluminum alloys, such as alloys containing 98% aluminum and 2% copper. As a consequence, because lower temperatures can be used for effective hole filling, the respective wafer or substrate containing the hole undergoes less thermal stress, which decreases the potential for damage to the structures, and ultimately the complete devices, being formed on and in the semiconductor material. On the other hand, high melting-point aluminum alloys, such as the Alxe2x80x94Cu alloy referenced above, possess superior electromagnetic and stress migration properties in comparison to low melting-point aluminum alloys and would thus be favored for use in contact/via fill processes if the disadvantages thereof could be reduced or eliminated.
Thus, it would be advantageous to provide an aluminum plug fill process which could be carried out at reduced temperatures and which also affords the superior electromagnetic and stress migration properties inherent in high melting-point aluminum alloys.
The present invention is directed to an improved method for filling contact holes or vias of semiconductor devices and the resulting structures. The improved method begins with insertion of the semiconductor wafer or other substrate of semiconductive material, having one or more contact holes or vias formed in an insulating layer overlying a wafer substrate, into a high-pressure heated chamber. A low-melting point base layer of aluminum material is then deposited over the insulating layer and into the contact holes or vias. During the deposition step, the wafer is heated up to the melting point of the aluminum material to reflow the same into the contact hole or via. Once deposition is completed and while maintaining the elevated temperature, the chamber is pressurized to force the aluminum material into the contact holes or vias and thus eliminate voids present therein under the aluminum material base layer. A second layer of material, comprising a metal or alloy to be used as a dopant source, is then deposited over a top surface of the deposited aluminum material base layer and allowed to diffuse into the aluminum material base layer in order to form a substantially homogenous aluminum alloy within the contact hole or via. The newly formed homogenous aluminum alloy possesses the desirable characteristics of the previously-mentioned high melting-point aluminum alloys, but without the associated difficulties and disadvantages of depositing such alloys in their preformed state. Formation of the homogenous aluminum alloy within the contact holes or vias of the wafer thus improves the strength, stress migration, and electromagnetic properties of the contacts or vias in a viable, economical manner easily applied to existing fabrication methodologies.